Maglevs by themselves are still too slow to really compete with air travel over long distances
As constituted now, maybe. What upper speed limit are you seeing? Because I see Mach 1 as mine. There's no reason I see a maglev can't do that. And I see even a 400mph maglev with advantages in city center to city center. It doesn't have quite that edge if there are a lot of stops involved, but that might mean two types of train (so to speak): an interurban/radial that stops a lot (& accelerates & decelerates much faster) & a long-distance (slower, perhaps, to peak speed, but it stays there much longer). (Notice, too, the acceleration/deceleration issue, which contributes to shorter trip time--& even a slow maglev beats conventional rail on that one.)
This also gets around many of the issues of grade, & energy inefficiency (tho, TBH, I can't see how trucking, in any way, is more energy efficient). As noted elsewhere, maglevs really do like straight lines, even more than HSR, so that's an issue that needs working out--but if you're doing HSR at all, it does, so...
BTW, lighter passenger cars enable better accel/decel, too, which help make HSR practical.
its value has to come from externalities like lowered pollution and road traffic, reduced city center traffic and parking requirements, reduced footprint compared to highways
Those all sound like really good reasons to be supporting it everywhere...& reasons to encourage use so it will actually make money.
Edit:
Something I'm not seeing addressed: taxes.
Rail companies don't get their construction & maintenance subsidized by the taxpayers like trucking companies & airlines do, & they have to pay taxes on the land under their rails, which truckers & airlines don't.
How much more profitable is passenger rail, HSR or not, if there's no tax on the rails? I've seen it said if passenger rail got even half what highways do in public money, it would make a profit.
Edit 2:
Now tell me how in the hell you are going to get the senators from places like Idaho and Montana to sign on to it when their constituents gain not dime one from it and yet are forced to pay for it.
Two ways. One's already been discussed: put it in a transportation bill where they're getting highway money already.
Two is the old-fashioned DC logroll: offer them pork.
No Senator will turn it down.
(And I should have thought to mention this long ago.
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